Embryonal carcinoma stem cells prvide a useful system cell lineage specification during early mammalian embryogenesis. However, a major problem in such studies is the lack of markers with which to recognize different cell lineages. Futher, the markers which are available are specific to genes which are utilized at later stages of differentiation. Progress in this system will requrire that markers are available for the different lineages formed not only at later stages of differentation where fully differentiatied cell types are present but also at the earliest stages when different lineages are first distinguishable. The primary objective of this study is to identify a set of cDNAs to messenger RNAs which are expressed cell type specifically in many of the different cell lineages which are formed during the differentiation of the pluripotent embryonal carcinoma stem cell line P19. Such cDNAs will be identified for cell lineages present at different times folowing the initiation of differentiation, from the earliest differentiation events through fully differentiated cell types. Identification of these cDNAs will be accomplished by taking advantage of the fact that the cell lineages into which P19 differentiates can be influenced by the conditions under which differentiation is induced. This allows the isolation of RNAs present at different stages of differentiation from cultures which contain largely non-overlapping cell types. Many of the differences in the message populations present in these different cultures will be cell type specific. To isolate cDNAs containing these sequences, we will take advantage of cDNA cloning and hybridization/selection technologies, developed in this laboratory, which allow much more efficient selection for sequences which differ between different cell populations than has previously been possible.